Trench-gated mosfet including schottky diode therein

ABSTRACT

Disclosed is a trench MOSFET, including: a trench gate structure having a gate electrode and a gate insulating film; an n-type diffusion layer formed to face the gate electrode via the gate insulating film at an upper portion of the trench; a p-type base layer formed to face the gate electrode via the gate insulating film at a lower portion than the upper portion; an n-type epitaxial layer locating to face the gate electrode via the gate insulating film at a further lower portion than the lower portion; a metal layer formed departing from the trench in parallel with a depth direction of the trench, penetrating the n-type diffusion layer and the p-type base layer, to reach the n-type epitaxial layer; and a p-type layer with higher impurity concentration than the p-type base layer, locating to be in contact with the p-type base layer and the metal layer.

CROSS-REFERENCE TO THE INVENTION

This application is a division of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 11/127,224, filed May 12, 2005,and claims the benefit of priority from the prior Japanese PatentApplication No. 2004-145265, filed on May 14, 2004 and the priorJapanese Patent Application No. 2005-112645, filed on Apr. 8, 2005; theentire contents of each application are incorporated herein byreference.

BACKGROUND

1. Field of the Invention

The present invention relates to a trench MOSFET including a gateelectrode having a trench gate structure, and particularly to the trenchMOSFET in which parallel schottky diodes are formed and housed between asource and a drain thereof.

2. Description of the Related Art

In a step down DC-DC converter, a MOSFET is used as a switching(chopping) element. During a period of current interruption from aprimary side to a secondary side by the switching, a structure to flowback the current by a flywheel diode is generally used so as not tointerrupt the current at a load side. However, in accordance with arequirement for a DC-DC converter having a low output voltage at theload side, a forward voltage drop of the above-stated diode cannot beignored as a loss. Consequently, a structure is also used, in which apassage between a source and a drain of another MOSFET (second MOSFET)is used instead of the diode, and it is turned on for the same period asthe period when the diode is conducting. As a typical MOSFET used forsuch a purpose, there is so-called a trench MOSFET having a trench gatestructure.

In the above-stated structure, it is difficult to control a gate voltageso as to turn on the second MOSFET for the completely same period as theperiod when the diode is conducting, and actually, it is used to have aperiod when both the MOSFET for chopping and the second MOSFET areturned off (dead time). During this dead time, the second MOSFETfunctions as a diode (pn junction diode) which is included as aparasitic element. The period is short, but the forward voltage dropthereof becomes a problem as the loss, all the same. Consequently, toreduce the forward voltage drop during the dead time, a structure inwhich schottky diodes are connected to the second MOSFET in parallel, isused.

Such a schottky diode is conceivable to be connected as a differentcomponent from the second MOSFET, but it is also conceivable to house inthe second MOSFET from points of view of advantages in a converterconfiguration. There are some points to be considered when it is housed.One of them is a cost. In this means, the schottky diode having astructure which does not occupy too many areas on a semiconductorsubstrate, and not making processes excessively complicated, isdesirable. Besides, it is also required to consider not to disturbfunctions as a basic schottky diode (for example, a low resistivity, alow leakage current, and so on) or functions as a transistor by removingthese factors giving influences to the cost. Incidentally, assemiconductor elements in which a schottky barrier is intentionallyformed within the semiconductor, for example, there are ones describedin the following respective Patent Documents.

[Patent Document 1] Japanese Patent Laid-open Application No. Hei11-251573

[Patent Document 2] Japanese Patent Laid-open Application No. 2003-17701

[Patent Document 3] U.S. Publication No. U.S.-A1-2003/0207538

SUMMARY

A trench MOSFET according to an aspect of the present invention,including: a gate electrode having a trench gate structure; a gateinsulating film formed to surround the gate electrode; an n-typediffusion layer formed to face the gate electrode via the gateinsulating film at an upper portion of the trench; a p-type base layerformed to face the gate electrode via the gate insulating film at alower portion than the upper portion of the trench; an n-type epitaxiallayer locating to face the gate electrode via the gate insulating filmat further lower portion than the lower portion of the trench; a metallayer formed departing from the trench in parallel with a depthdirection of the trench, penetrating the n-type diffusion layer and thep-type base layer to reach the n-type epitaxial layer; and a p-typelayer with higher impurity concentration than the p-type base layer,locating to be in contact with the p-type base layer and the metallayer.

Further, a trench MOSFET according to another aspect of the presentinvention, including: a gate electrode having a trench (first trench)gate structure; a gate insulating film formed to surround the gateelectrode; an n-type diffusion layer formed to face the gate electrodevia the gate insulating film at an upper portion of the first trench; ap-type base layer formed to face the gate electrode via the gateinsulating film at a lower portion than the upper portion of the firsttrench; a second trench locating to face the first trench via the p-typebase layer, respectively having an insulating film at a side wallsurface and a p-type semiconductor layer at a bottom surface, and inwhich a conductor layer is embedded; an n-type epitaxial layer locatingto face the gate electrode via the gate insulating film at a furtherlower portion than the lower portion of the first trench, and alsolocating at an opposite side of the second trench from a side in whichthe p-type base layer is located; and a metal layer formed to contact atan upper surface of the n-type epitaxial layer at the opposite sideportion of the second trench from the side where the p-type base layeris located.

Further, a trench MOSFET according to still another aspect of thepresent invention, including: a gate electrode having a trench gatestructure; a gate insulating film formed to surround the gate electrode;an n-type diffusion layer formed to face the gate electrode via the gateinsulating film at an upper portion of the trench; a p-type base layerformed to face the gate electrode via the gate insulating film at alower portion than the upper portion of the trench; a p-typesemiconductor layer formed to face the trench via the p-type base layer,and to reach deeper than the trench; an n-type epitaxial layer locatingto face the gate electrode via the gate insulating film at a furtherlower portion than the lower portion of the trench, and also locating atan opposite side of the p-type semiconductor layer from a side in whichthe p-type base layer is located; and a metal layer formed to contact atan upper surface of the n-type epitaxial layer at the opposite sideportion of the p-type semiconductor layer from the side where the p-typebase layer is located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of a circuit when a trench MOSFETaccording to each embodiment of the present invention is used.

FIG. 2 is a sectional view schematically showing a structure of a trenchMOSFET according to an embodiment of the present invention;

FIG. 3 is a sectional view schematically showing a structure of a trenchMOSFET according to another embodiment of the present invention.

FIG. 4A and FIG. 4B are process charts showing manufacturing processesof a trench MOSFET according to still another embodiment of the presentinvention in schematic cross sections.

FIG. 5 is a subsequent view of FIG. 4B, and is a process chart showingthe manufacturing process of the trench MOSFET according to stillanother embodiment of the present invention in schematic cross section.

FIG. 6 is a sectional view schematically showing a structure of a trenchMOSFET according to yet another embodiment of the present invention.

FIG. 7 is a sectional view schematically showing a structure of a trenchMOSFET according to yet another embodiment of the present invention.

FIG. 8 is a sectional view schematically showing a structure of a trenchMOSFET according to yet another embodiment of the present invention.

FIG. 9 is a sectional view schematically showing a structure of a trenchMOSFET according to yet another embodiment of the present invention.

FIG. 10 is a sectional view schematically showing a structure of atrench MOSFET according to yet another embodiment of the presentinvention.

FIG. 11 is a sectional view schematically showing a structure of atrench MOSFET according to yet another embodiment of the presentinvention.

FIG. 12 is a sectional view schematically showing a structure of atrench MOSFET as a comparative example.

DETAILED DESCRIPTION Description of Embodiments

Embodiments of the present invention will be described with reference tothe drawings, but these drawings are provided only for an illustrativepurpose and by no means are intended to limit the present invention.

As for a trench MOSFET according to a mode of the present invention, inthe trench MOSFET in which a p-type base layer facing a gate insulatingfilm becomes a channel, there is a metal layer which penetrates ann-type diffusion layer, and further a p-type base layer, to reach ann-type epitaxial layer becoming a part of a drain region. Consequently,a junction from this metal layer to the n-type epitaxial layer becomesto be a schottky junction, and formed is a schottky diode having thisjunction whose forward direction is from the metal layer to the n-typeepitaxial layer. Besides, this metal layer locates departing from thetrench in parallel with a depth direction of the trench, and therefore,a planer area occupation thereof can be eliminated as same degree as thetrench.

Consequently, in consideration with a cost, it is possible to house theschottky diode having the small area occupation on a semiconductorsubstrate. Incidentally, the reason why a p-type layer with a higherimpurity concentration than the p-type base layer is provided at theposition to be in contact with the p-type base layer and the metal layeris to certify that a conductivity between the metal layer and the p-typebase layer is performed via an ohmic junction, and to secure anavalanche tolerance in a state that a backward voltage is applied to theschottky diode. It is a removal of a disturbing factor of functions.

As a form in this mode, a depth of the metal layer can be set to beshallower than the depth of the trench. Because it is structurallypossible for the metal layer to reach the n-type epitaxial layer even ifit is shallower than the trench. Incidentally, when a recessed portionto embed the metal layer is formed by the simultaneous process as theformation of the trench, the depths of them are to be the same degree.

Besides, as another form, the depth of the metal layer can be set to beshallower than the depth of the deepest portion of the p-type baselayer. For example, when a thickness of the p-type base layer at theother region is set to be deeper than around the metal layer, this formis realized.

Besides, as still another form, the depth at the deepest portion of thep-type base layer can be set to be deeper than the depth of the trenchand the depth of the metal layer. For example, the thickness of thep-type base layer at the other region is set to be deeper than aroundthe trench and the metal layer, this form is realized.

Besides, as yet another form, an n-type layer with higher impurityconcentration than the n-type diffusion layer locating sandwichedbetween the n-type diffusion layer and the metal layer, may further beincluded. For example, it is a form in which a contact to the n-typediffusion layer to be a source region from a source electrode layer isperformed via the metal layer and the high impurity concentration n-typelayer. Such high impurity concentration n-type layer can be selectivelyformed by using, for example, a high acceleration ion implantationmethod.

Besides, as for a trench MOSFET according to another mode, in the trenchMOSFET in which a p-type base layer facing a gate insulating filmbecomes a channel, there is a trench (second trench) different from atrench (first trench) formed to face via a p-type base layer the firsttrench in which a gate electrode is embedded. The second trench each hasan insulating film on a side wall surface, and a p-type semiconductorlayer on a bottom surface, and further, an opposite side from a sidewhere a p-type base layer is located, is an n-type epitaxial layer. Atan upper surface of the n-type epitaxial layer of this portion, a metallayer is formed to contact. Consequently, a junction from this metallayer to the n-type epitaxial layer becomes to be a schottky junction,and formed is a schottky diode having this junction whose forwarddirection is from the metal layer to the n-type epitaxial layer.Besides, this metal layer is provided to be adjacent to the secondtrench, and therefore, a planar area occupation can be made smaller.

Consequently, in consideration with a cost, it is possible to house theschottky diode having the small area occupation on a semiconductorsubstrate. Incidentally, the reason why the bottom surface of the secondtrench is the p-type semiconductor layer, is to easily form a depletionlayer at around the n-type epitaxial layer in a state when a backwardvoltage is applied to the schottky junction, and to suppress a leakagecurrent of the schottky junction into a small level. It is a removal ofa disturbing factor of functions.

As a form of this mode, the conductor layer of the second trench may bep-type polycrystalline silicon. It is a form to use polycrystallinesilicon as an electric conductor. The reason why it is a p-type is toprevent a contact with the p-type semiconductor layer formed at thebottom portion of the second trench from being a pn junction. Besides,p-type polycrystalline silicon has a large work function difference withsilicon, which is different from n-type polycrystalline silicon, andtherefore, in the n-type epitaxial layer facing via the insulating filmof the second trench, a depletion layer is easy to be spread from thesecond trench. Consequently, the leakage current of the schottky diodecan further be suppressed, and the disturbing factor of the functionscan further be removed.

Besides, as another form, the conductor layer of the second trench maybe a metal. It is a form to use the metal as the electric conductor.

Besides, as still another form, the conductor layer of the second trenchmay be in contact and conducted with the metal layer at an upper surfacethereof. It is an example of a structure to keep electric potentials ofthe conductor layer in the second trench and the metal layer to be thesame.

Besides, as yet another form, the insulating film at the side wallsurface of the second trench may be extendedly provided at the uppersurface of the conductor layer of the second trench to isolate betweenthe conductor layer and the metal layer. In this case, it is alsopreferable to keep the electric potentials of the conductor layer in thesecond trench and the metal layer to be the same, and for this purpose,a contact to the conductor layer is to be provided at an end portion ina longitudinal direction of the second trench.

Besides, as yet another form, the p-type semiconductor layer at thebottom surface of the second trench may be in contact with the p-typebase layer. In accordance with a degree of a formation depth of thesecond trench relative to a formation thickness of the p-type baselayer, or the degree of the formation thickness of the p-typesemiconductor layer at the bottom surface of the second trench, thisform is realized.

Besides, as yet another form, the gate insulating film may be integratedwith the insulating layer protruding from the first trench into an upperportion. It is an example of a shape of the insulating layer to be thegate insulating film.

Besides, as for a trench MOSFET according to still another mode of thepresent invention, in the trench MOSFET in which a p-type base layerfacing a gate insulating film becomes a channel, there is a p-typesemiconductor layer formed to face a trench in which a gate electrode isembedded, via the p-type base layer, deeper than the trench. An oppositeside of this p-type semiconductor layer from a side where the p-typebase layer is located, is an n-type epitaxial layer. At an upper surfaceof the n-type epitaxial layer at this portion, a metal layer is formedto contact. Consequently, a junction from this metal layer to the n-typeepitaxial layer becomes to be a schottky junction, and formed is aschottky diode having this junction whose forward direction is from themetal layer to the n-type epitaxial layer. Besides, this metal layer isprovided to be adjacent to the p-type semiconductor layer, andtherefore, a planar area occupation can be made smaller.

Consequently, in consideration with a cost, it is possible to house theschottky diode having the small area occupation on a semiconductorsubstrate. Incidentally, the p-type semiconductor layer is formed deeperthan the trench, and therefore, a formation of a depletion layer ataround the n-type epitaxial layer in a state when a backward voltage isapplied to the schottky junction is spread more largely, and it ispossible to suppress a leakage current of the schottky junction into asmall level. It is a removal of a disturbing factor of functions.

As a form of this mode, the gate insulating film may be integrated withthe insulating layer protruding from the trench into the upper portion.It is an example of a shape of the insulating layer to be the gateinsulating film.

Besides, as another form, the p-type semiconductor layer may have atrench structure. Namely, it is a form in which the trench is formed,and thereafter, the p-type semiconductor layer is embedded insidethereof. Other than this form, such p-type semiconductor layer can beformed by an ion implantation.

Besides, as still another form, the trenches may be formed in plural inparallel with each other, and a missing of the trench exists at aformation pitch thereof. It is a form to form the schottky junction atthe missing portion of the trench.

Besides, as yet another form, the trenches may be formed in plural inparallel with each other, and the formation pitch thereof is almostuniform. It is a form to form the p-type semiconductor layer and theportion of the schottky junction between the trenches formed with theuniform pitch. Herewith, it becomes possible to mixedly house theschottky diode without expanding cell pitches, and an on-resistance asthe MOSFET does not increase because the area as the MOSFET does notincrease.

Thinking of the above, hereinafter, embodiments of the present inventionwill be described with reference to the drawings. FIG. 1 is a viewshowing an example of a circuit (DC-DC converter) using a trench MOSFETaccording to respective embodiments of the present invention. The trenchMOSFET used in FIG. 1 is a transistor Q2, and at first, a function ofthe transistor Q2 is described.

FIG. 1 is a circuit (DC-DC converter) in which a high input voltage (forexample, 17 V) is applied between input terminals (primary side) at aleft side in the drawing and obtains a low output voltage (for example,1.5 V) between output terminals (secondary side) at a right side in thedrawing. These voltage ratio can be set by a duty ratio of a switching(chopping) at a transistor Q1. When the transistor Q1 is turned on, acurrent flows from the primary side to the secondary side. When thetransistor Q1 is turned off, the current at the second side flows backin a direction as it is, and therefore, the transistor Q2 is turned on.A set of an inductor L and a capacitor C is a low pass filter. Gateinput signals of approximately inverting phase generated at a gate inputsignal generation circuit 10 are supplied to the respective gates of thetransistors Q1 and Q2, to control the turning on/off of the transistorsQ1 and Q2.

Normally, it is possible to flow back the current when the transistor Q1is turned off, by providing a diode D, and the transistor Q2 is notnecessary. However, when the output voltage required at the secondaryside is low, a forward voltage drop of the diode D becomes large not tobe ignored as a loss, and it is necessary to make the voltage lower.Consequently, the transistor Q2 which is turned on/off approximately inthe inverting phase with the turning on/off of the transistor Q1 isprovided as shown in the drawing.

Strictly speaking, a setting of the phase to turn on/off the transistorsQ1 and Q2 is performed to provide a short period when the both areturned off, (a negative pulse width toward the Q2 is a little wider thana positive pulse width toward the Q1 shown by the pulse in the drawing).Herewith, it is possible to prevent an occurrence of a short-circuitperiod at the primary side. However, a diode as a structural parasiticelement is turned on at the normal transistor Q2, by the occurrence ofthe period when both of the transistors Q1 and Q2 are turned off (deadtime). The forward voltage drop of this diode cannot be ignored as aloss, none the less. Consequently, the transistor Q2 in each embodimenthas the schottky diodes structurally built-in in parallel between asource and a drain. Herewith, it is possible to effectively lower thevoltage between the source and the drain of the transistor Q2 during thedead time. Besides, as it is described in the following, it isconsidered to secure a low cost and a basic functionality as thetransistor Q2.

FIG. 12 is a sectional view schematically showing a structure of atrench MOSFET as a comparative example. At first, this comparativeexample is described. As shown in FIG. 12, this trench MOSFET has adrain electrode layer 21, an n⁺ layer 22, an n-type epitaxial layer 23,a p-type base layer 24, an n-type diffusion layer 25, a source electrodelayer 26, gate electrodes 27, and gate insulating films 28.Incidentally, sizes in a horizontal direction in the drawing and in avertical direction to the paper in the drawing are, for example, from 5mm to 7 mm. The same pattern in the drawing is repeated in thehorizontal direction, and it has the same cross-sectional structure withthe drawing in the vertical direction to the paper. It is assumed that acurrent rating is from 10 A to 20 A, and a withstand voltage (backwardvoltage) is from 20 V to 30 V, in this comparative example and in thefollowing respective embodiments at present, but they are not limited tothese.

The drain electrode layer 21 is a conductive layer stacked on a sidecorresponding to a rear surface of a semiconductor substrate, namely onthe n⁺ layer 22. For example, it can be formed from a lower side (n⁺layer 22 side) as a stacked layer of a vanadium layer as a barrierlayer, a nickel layer being a base metal, and further, a gold layer toprevent a corrosion. The n⁺ layer 22 is a semiconductor layer having ahigh n-type impurity concentration, and it is in ohmic junction with thedrain electrode layer 21. The n-type epitaxial layer 23 locating at anopposite side of the n⁺ layer 22 with the drain electrode layer 21, isthe semiconductor layer made by an epitaxial growth having a lowerimpurity concentration compared to the n⁺ layer 22. The n⁺ layer 22 andthe n-type epitaxial layer 23 become to be a drain region as the MOSFET.

The p-type base layer 24 is the semiconductor layer formed, for example,by implanting and diffusing a p-type impurity into the n-type epitaxiallayer 23. A region facing the later-described gate electrodes 27 via thegate insulating film 28 becomes to be a channel as the MOSFET. Then-type diffusion layer 25 is the semiconductor layer formed, forexample, by implanting and diffusing the n-type impurity into the n-typeepitaxial layer 23. The n-type diffusion layer 25 becomes to be thesource region as the MOSFET. The source electrode layer 26 is aconductive layer formed on the n-type diffusion layer 25. For example,it can be formed from the lower side (n-type diffusion layer 25 side) asthe stacked layer of a titanium layer as the barrier layer and analuminum layer being a base metal.

The gate electrode 27 is a semiconductor conductive layer composed of,for example, polycrystalline silicon embedded inside of a trench formedpenetrating the n-type diffusion layer 25 and the p-type base layer 24to cut into the n-type epitaxial layer 23. The trench has a shapeextending in the vertical direction to the paper in the drawing. Asshown in the drawing, a plenty of trenches are formed in parallel, andpitches thereof can be set, for example, from 1 μm to 2 μm. Therespective gate electrodes 27 having trench gate structures areconnected to, for example, a common electrode of aluminum at both endportions thereof, and they become to be one terminal (gate terminal).The gate insulating film 28 is, for example, an oxide film formed tosurround the gate electrodes 27.

The metal layer 29 is the conductive layer formed departing from thetrench in parallel with the depth direction of the trench, penetratingthe n-type diffusion layer 25 and the p-type base layer 24, and to cutinto the n-type epitaxial layer 23. A formation method thereof is, forexample, to form other trench than the trench for the gate electrode 27formation, at the n-type diffusion layer 25, the p-type base layer 24,and the n-type epitaxial layer 23, and the metal layer is embeddedinside thereof. For example, titanium can be used as a material thereof.

According to the trench MOSFET having a constitution as described above,in addition to a normal MOSFET constituted by the source region of then-type diffusion layer 25, the channel of the p-type base layer 24, thedrain region of the n-type epitaxial layer 23, the gate insulating film28, and the gate electrodes 27, formed is a plenty of schottky diodesmade by the junction between the metal layers 29 and the n-typeepitaxial layer 23 to be housed. This schottky diode is in a forwarddirection from the source electrode layer 26 side to the drain electrodelayer 21 side, and it can be a good bypass route of the current when theabove-stated MOSFET is turned off. Besides, a switching of the currentwith the MOSFET portion is quick and a desirable flow back of thecurrent is possible compared to the case when an external schottky diodeis provided. When the external schottky diode is provided, an inductorcomponent is included at a conductor portion, and therefore, theswitching of the current becomes late.

Further, the metal layers 29 are formed by using the regions between thetrenches of the gate electrodes 27 to make this as a metal electrodeside of the schottky diode, and therefore, total area as a device isseldom increased. Besides, a current density is diffused as a whole,since a plurality of schottky diodes are formed, and therefore, there isan advantage that a factor to increase the forward voltage does notoccur.

Incidentally, as it is obvious by referring to FIG. 1, when the drainelectrode layer 21 side is positive, and the source electrode layer 26side is negative, the transistor Q1 is turned on and the trench MOSFET(Q2) according to this embodiment is turned off. Besides, the backwardvoltage is applied to the schottky diodes formed inside. At this time,in the n-type epitaxial layer 23, depletion layers are spreading fromaround the p-type base layer 24, and further, from around the metallayers 29. However, formation regions of the depletion layers at aroundside surfaces of the metal layers 29 are relatively small. Consequently,a leakage current (a backward current of the schottky diode) may beconcerned according to usage conditions thereof. In such a case, it isconceivable that the material of the metal layer 29 is changed to usethe material in which the leakage current becomes small. For example,platinum or platinum silicide is usable.

As described above, the trench MOSFET as the comparative example shownin FIG. 12 achieves the built-in of the schottky diodes, butstructurally, there is a case when the leakage current is concerned.Further, it is probable that there are cases when enough specificationscannot be secured from points of views of a withstand voltage at theportion functioning as the MOSFET, and an avalanche tolerance. Thosebecome more obvious by comparison with respective embodiments describedbelow.

FIG. 2 is a sectional view schematically showing a structure of a trenchMOSFET according to an embodiment of the present invention. In FIG. 2,the same reference numerals and symbols in FIG. 12 are used to designatethe same and corresponding elements, and the detailed descriptionthereof will not be given except for the case when there is something toadd.

In this embodiment, it is upgraded so that schottky junction regions ofmetal layers 39 and an n-type epitaxial layer 23 are formed only atlower surfaces of the metal layers 39. Consequently, a leakage currentwhen a backward voltage is applied can be made small. Namely, depletionlayers spreading from around p-type base layers 34 into the n-typeepitaxial layer 23 when the backward voltage is applied, cover junctionportions with the metal layers 39 easily, and a state in which theleakage current is made smaller can be made. Besides, since the areaswhere the gate electrodes 27 and the epitaxial layer 23 are facing viathe gate insulating films 28 are decreasing, capacities between gatesand drains are decreased, and there is also an effect to improve anoperation speed (switching speed).

N-type diffusion layers 25 a have the same function as the n-typediffusion layers 25 in FIG. 12, but high concentration p-type layers 34a are formed at a part of the region thereof, and they are injunctionwith the metal layers 39. The high concentration p-type layers 34 a canbe formed, for example, discontinuously (at intervals) in a verticaldirection to the paper in the drawing. Herewith, the metal layers 39 canrespectively secure ohmic junctions with n-type semiconductors via then-type diffusion layers 25 a and with p-type semiconductors (p-type baselayer 34) via the high concentration p-type layers 34 a. Consequently,an unintended schottky junctions between the metal layers 39 and thesemiconductor layers are prevented, and herewith, an occurrence of aparasitic transistor can be prevented, to increase an avalanchetolerance.

The metal layer 39 is integrally formed with the source electrode layer36. To form this, for example, a titanium layer as a barrier layer isformed inside of the trench and at an upper surface, and thereafter, analuminum layer is formed to embed inside of the trench and to cover theupper surface.

To make a regional relation between the trenches of the gate electrodes27, the trenches of the metal layers 39, and the p-type base layers 34as shown in the drawing, for example, the following process can beperformed. At first, a p-type layer to be a base of the p-type baselayer 34 is formed at a little deep position of the n-type epitaxiallayer 23 by an ion implantation. Next, the trenches for the metal layers39 are formed penetrating this p-type layer to cut into the n-typeepitaxial layer 23, and an n-type impurity is implanted into a bottomthereof. Subsequently, the impurity of the p-type layer is diffused,then the p-type base layer 34 to reach the bottom portions of thetrenches of the metal layers 39, is formed. At this time, in the regionwhere the n-type impurity is not implanted, the p-type region is morespread, and therefore, the p-type base layers 34 projecting downwardbetween the trenches of the metal layers 39 as shown in the drawing areformed. After that, the trenches for the gate electrodes 27 are formedpenetrating the p-type base layer 34 to cut into the n-type epitaxiallayer 23.

Next, a trench MOSFET according to another embodiment of the presentinvention is described with reference to FIG. 3. FIG. 3 is a sectionalview schematically showing a structure of the trench MOSFET according toanother embodiment of the present invention. In FIG. 3, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

In this embodiment, to secure ohmic junctions of metal layers 39 andp-type semiconductors, high concentration p-type layers 34 b are locatedat a part sandwiched by regions of the p-type base layers 34 and themetal layers 39. The function thereof is the same as the highconcentration p-type layer 34 a in the embodiment shown in FIG. 2.However, in this embodiment, it has a structure in which junction areasbetween the high concentration p-type layers 34 b and the metal layers39 can easily be earned in a depth direction. Besides, it is not able tooccur that the regions in the n-type diffusion layers 25 a are madesacrificed as in the embodiment shown in FIG. 2, and therefore, pitchesbetween the gate electrodes 27 can be set smaller. This is convenientfor making an area narrower and a cost lower as a device.

Incidentally, formations of the high concentration p-type layers 34 b asshown in the drawing can be made, for example, by selectivelyimplanting, in plan view, a p-type impurity into a predetermined depthof the p-type base layers 34. For this purpose, for example, a highacceleration ion implantation method can be used.

Next, a trench MOSFET according to still another embodiment of thepresent invention is described with reference to FIG. 4A, FIG. 4B, andFIG. 5. FIG. 4A, FIG. 4B, and FIG. 5 are process charts showingmanufacturing processes of the trench MOSFET according to still anotherembodiment of the present invention in schematic cross sections, and theprocesses progress in sequence of FIG. 4A, FIG. 4B, and FIG. 5. In FIG.4A, FIG. 4B, and FIG. 5, the same reference numerals and symbols alreadyshown are used to designate the same and corresponding elements, and thedetailed description thereof may not be given.

In this embodiment, the trenches to form the metal layers 39 (29) andthe trenches to form the gate electrodes 27 in the above-statedembodiment are formed by one simultaneous process, and the manufacturingprocess thereof can be simplified. Further, the advantage to make thepitches between the gate electrodes narrower in the embodiment shown inFIG. 3 and the advantages of the suppress of the leakage current of theschottky diodes, the reduction of the capacity between the gate and thedrain, the increase of the avalanche tolerance, and so on are took over.

At first, with reference to FIG. 4A, phosphorus is high energy ionimplanted into near a surface of an n-type epitaxial layer 23, boron ishigh energy ion implanted into a deep portion respectively, to formn-type layers 55 and p-type layers 54. Next, trenches 61 for metallayers and the trenches 61 for gate electrodes are formed at the sametime. Phosphorus is then ion implanted into a bottom portion of thetrenches 61, and regions with high n-type concentration are formed.

Next, with reference to FIG. 4B, an insulating film 58 is formed byperforming an oxidation including inside walls of the trenches 61. Atthis time, at the bottom portion of the trenches 61, the insulating film58 is thickly formed because of the region having the higher n-typeconcentration. Besides, the impurities within the n-type layers 55 andthe p-type layers 54 are diffused, and the regions become large inthickness directions, to be n-type diffusion layers 55 a and p-type baselayers 54 a. In particular, the p-type base layers 54 a are more spreadat the regions where the n-type impurity implanted into the bottomportion of the respective trenches 61 does not exist, and therefore,they become to be the p-type base layers 54 a projecting downwardbetween the respective trenches 61 as shown in the drawing.

Next, a polycrystalline silicon layer 57 is formed on an upper surfacesincluding insides of the respective trenches 61. Subsequently, as shownin the drawings, as for the trenches 61 in which the metal layers areembedded later, high concentration n-type layers 55 b are formed at thedepths of upper portions of the n-type diffusion layers 55 a, and highconcentration p-type layers 54 b are formed at the depths of the p-typebase layers 54 a, respectively by means of the ion implantations. Thehigh concentration n-type layers 55 b and the high concentration p-typelayers 54 b are formed for ohmic contacts with the metal layers embeddedinsides of the trenches 61 later. These layers are not necessarily to beformed under a condition in which ohmic junctions with the n-typediffusion layers 55 a and the p-type base layers 54 a are possible to beformed without there layers.

Next, phosphorus is doped into the formed polycrystalline silicon layer57, and further, the surface thereof is smoothed. The smoothed surfaceis then oxidized, and as shown in FIG. 5, gate electrodes 57 a areformed in isolation. At this time, a gate insulating film 58 a is formedwith a part of the insulating film 58 and an oxide insulating film at asurface layer of the polycrystalline silicon layer 57. Next, thetrenches are dug again alternately so as to remove all of thepolycrystalline silicon layer 57 and the insulating film 58 to obtainthe trenches for forming metal layers 59. At this time, a part of thehigh concentration n-type layers 55 b and the high concentration p-typelayers 54 b are removed along the trench shape to become highconcentration n-type layers 55 c and high concentration p-type layers 54c as final shapes.

Next, the metal layers 59 and a source electrode layer 56 are formed bydepositing a metal layer at an upper surface including insides of thedug again trenches. Besides, a drain electrode layer 21 is formed on ann⁺ layer 22 corresponding to a rear surface of the semiconductorsubstrate. As materials of the metal layers 59 and the source electrodelayer 56, for example, tungsten, or aluminum using titanium or platinumsilicide as a barrier layer can be used.

Next, a trench MOSFET according to yet another embodiment of the presentinvention is described with reference to FIG. 6. FIG. 6 is a sectionalview schematically showing a structure of the trench MOSFET according toyet another embodiment of the present invention. In FIG. 6, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

The structure of the trench MOSFET of this embodiment is almost the sameas the one shown in FIG. 3. The difference is that formation regions ina depth direction of p-type base layers 34 c are different from the oneshown in FIG. 3. To make such a structure, the p-type base layers 34 care to be formed by diffusing p-type layers in the depth direction aftertrenches for gate electrodes 27 and the trenches for metal layers 39 areformed, as same as the embodiment referring to the above-stated FIG. 4A,FIG. 4B, and FIG. 5.

Hereinabove, the comparative example and the four embodiments aredescribed, and speaking from point of view of regions, in the modes ofFIG. 12, FIG. 2, and FIG. 3, the depths of the metal layers 29 and 39are shallower than the depths of the trenches of the gate electrodes 27.Besides, in the modes of FIG. 2, FIG. 3, FIG. 5, and FIG. 6, the depthsof the metal layers 39 and 59 are shallower than the depths of thedeepest portions of the p-type base layers 34, 54 a, and 34 c. Further,in the modes of FIG. 5 and FIG. 6, the depths of the deepest portions ofthe p-type base layers 54 a and 34 c are deeper than the depths of thetrenches of the gate electrodes 57 a and 27, and the depths of the metallayers 59 and 39.

Next, a trench MOSFET according to yet another embodiment of the presentinvention is described with reference to FIG. 7. FIG. 7 is a sectionalview schematically showing a structure of the trench MOSFET according toyet another embodiment of the present invention. In FIG. 7, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

As shown in FIG. 7, this trench MOSFET has p-type base layers 74, n-typediffusion layers 75, a source electrode layer 76, gate electrodes 77,and gate insulating films 78, in addition to a drain electrode layer 21,an n⁺ layer 22, and an n-type epitaxial layer 23. Besides, there aresecond trenches facing the gate electrodes 77, and they have fillingstructures in which there are an insulating film 70 a on a side wall anda p-type semiconductor layer 70 b at a bottom portion inside thereof,and there is a conductor layer 70 c in further inside. Besides, a partof an upper surface of the p-type base layer 74 is a high concentrationp-type layer 74 a having high concentration of a p-type impurity, andthereby, a certain ohmic junction with the source electrode layer 76 canbe obtained.

The respective functions of the p-type base layer 74, the n-typediffusion layer 75, the source electrode layer 76, the gate electrode77, and the gate insulating film 78 are respectively the same as thoseof the p-type base layers 34, 54 a, and 34 c, the n-type diffusionlayers 25 a, 25, and 55 a, the source electrode layers 36 and 56, thegate electrodes 27 and 57 a, and the gate insulating films 28 and 58 ain the embodiments cited above. However, in this embodiment, the gateinsulating film 78 surrounding the gate electrode 77 in the trench, isintegrated with the insulating film protruding from the trench into anupper portion. To obtain such an insulating film structure, the gateelectrode 77 and the gate insulating film 78 in the trench are formed,and thereafter, for example, the insulating film is accumulated on awhole surface, and then, performed is a selective etching so as to leavethe insulating film at the region as shown in the drawing.

Besides, the second trench composed of the conductor layer 70 c, and soon, is facing the p-type base layer 74 at one side, but at an oppositeside, the p-type base layer 74 is not formed, and the n-type epitaxiallayer 23 is spreading up to the surface of the semiconductor region. Toform the p-type base layer 74 at the selective region like this, forexample, the second trench is formed, and thereafter, a p-type impurityis implanted via a mask formed at a predetermined region and isdiffused. Besides, as for the second trench, as it is different from thetrench for the gate electrode 77, there is no facing with the n-typediffusion layer 75 on any side. To form the n-type diffusion layer 75 atsuch a selective region, for example, a mask formed at the predeterminedregion may also be used.

The p-type semiconductor layer 70 b at the bottom portion of the secondtrench can be formed by forming the trench, forming an oxide film at aninner wall and a bottom surface thereof, thereafter, selectivelyremoving the insulating film at the bottom surface, and further,implanting the p-type impurity into the bottom portion of the trench.This p-type semiconductor layer 70 b may be formed to be in contact withthe p-type base layer 74. A material of conductor layer 70 c fillinginside may be, for example, p-type polycrystalline silicon. When it isan n-type, the contact with the p-type semiconductor layer 70 b becometo be a pn junction, and therefore, it is not preferable. Instead ofp-type polycrystalline silicon, a metal (metal silicide) can be used.The conductor layer 70 c is in contact with the source electrode layer76 at an upper surface thereof, but a part (for example, at intervals)in the vertical direction to the paper in the drawing may be in contactwith the source electrode layer 76 without being contacted by the wholetrench.

In the trench MOSFET having the structure as described above, thecontact portion of the n-type epitaxial layer 23 spreading up to thesurface of the semiconductor region and the source electrode layer 76being the metal layer becomes to be a schottky junction. Here, at thebottom portion of the trenches adjacent to the n-type epitaxial layer 23in the region of the schottky junction, the p-type semiconductor layers70 b play a roll to eliminate the leakage current in this schottkyjunction. Namely, in a state when a backward voltage is applied to theschottky junction, the depletion layers are formed at the n-typeepitaxial layer 23 around the p-type semiconductor layers 70 b as shownin the drawing, a carrier concentration taking a charge of the currentis lowered, a contribution of an electric field to around an interfaceof the schottky junction is reduced, and therefore, the leakage currentcan be reduced.

In the drawing, one unit is shown in the horizontal direction, and inthis case, two second trenches (namely, one schottky diode) are providedfor one trench of the gate electrode 77. However, it is not limited tothis, and a layout in which two or more trenches of the gate electrode77 are adjacent, and the two or more trenches each has one schottkydiode, may be acceptable.

Next, a trench MOSFET according to yet another embodiment of the presentinvention is described with reference to FIG. 8. FIG. 8 is a sectionalview schematically showing a structure of the trench MOSFET according toyet another embodiment of the present invention. In FIG. 8, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

In this embodiment, a point is different that the gate insulating film78 in the embodiment shown in FIG. 7 becomes to be a gate insulatingfilm 78 a which is not integrated with the insulating film protrudingoutside of the trench. Further, as for the second trenches, a point isdifferent that an upper surface of conductor layer 70 ca is covered withan insulating film 70 aa in the same mode with the gate electrode 77,but the rest are basically same.

The upper surface of the conductor layer 70 ca is covered with theinsulating film 70 aa for intending to form both upper surface portionsof the gate insulating film 78 a and the insulating film 70 aa at thesame time. The formation of the upper surface portions of theseinsulating films are performed by, for example, oxidizing the uppersurface portions of the embedded gate electrode 77 and the conductorlayer 70 ca at the same time to make them change into the insulatingfilms. The material of the conductor layer 70 ca may be, for example,p-type polycrystalline silicon or a metal, and to fix an electricpotential thereof, for example, conductive paths are formed at both endportions in the vertical direction to the drawing so as to be in conductwith the source electrode layer 76.

Next, a trench MOSFET according to yet another embodiment of the presentinvention is described with reference to FIG. 9. FIG. 9 is a sectionalview schematically showing a structure of the trench MOSFET according toyet another embodiment of the present invention. In FIG. 9, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

In this embodiment, a point as follows is a large difference from theabove-stated embodiments, that p-type semiconductor layers 80 are formedin wall state deeper than trenches for gate electrodes 77, between thetrenches in which the gate electrodes 77 are embedded, and between thetrench in which the gate electrode 77 is embedded and a region where aschottky junction is formed. According to such a structure, advantagesas a MOSFET such as a high withstand voltage, and a low on-resistance,can further be obtained. Here, an n-type epitaxial layer 73 may have ahigher impurity concentration than the n-type epitaxial layer 23 in theabove-stated respective embodiments.

The p-type semiconductor layers 80 which are adjacent to regions wherethe n-type epitaxial layer 73 is exposing to an upper surface of asemiconductor region, have higher degrees to form depletion layers inthe adjacent n-type epitaxial layer 73 from point of view of a shapethereof, than, for example, the p-type semiconductor layers 70 b in theembodiment shown in FIG. 7. Consequently, even if the n-type epitaxiallayer 73 has an impurity of high concentration, it is possible to reducethe leakage current of the schottky diode.

To form the p-type semiconductor layers 80 as shown in the drawing, forexample, a method to form trenches, and fill inside thereof with p-typesemiconductor layers, or a method to implant a p-type impurity into then-type epitaxial layer 73 while changing implanting energies, can beadopted. A formation depth thereof depends on a withstand voltagespecification, and for example, is approximately 10 μm or less when thewithstand voltage is 100 V, and it is formed deeper according tobecoming high withstand voltage.

In case of this embodiment, a ratio between the number of the trenchesof the gate electrode 77 and the number of schottky diodes to be formed,can be differed largely according to a layout thereof, as same as theembodiment shown in FIG. 7.

Next, a trench MOSFET according to yet another embodiment of the presentinvention is described with reference to FIG. 10. FIG. 10 is a sectionalview schematically showing a structure of the trench MOSFET according toyet another embodiment of the present invention. In FIG. 10, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

In this embodiment, as shown in the drawing, a width of a region where aschottky diode is formed is saved. Namely, in the embodiment shown inFIG. 9, one of the trenches for the gate electrode 77 is sacrificed, andthereby, one schottky diode region is created. However, in thisembodiment, the schottky diode region is formed without sacrificing anytrenches for the gate electrode 77.

More concretely, two p-type semiconductor layers 80 a different from thep-type semiconductor layer 80 between the trenches of the gate electrode77, are formed in wall state to face each other at the region where theschottky diode is formed between the trenches of the gate electrode 77.It is preferable that depletion layers around the p-type semiconductorlayers 80 a formed at the n-type epitaxial layer 73 are further easy tobe disturbances of a formation of a flowing path of the leakage currentof the schottky diode, according to such a characteristic in shape.Incidentally, at an upper surface of the p-type semiconductor layers 80a, high concentration p-type layers 74 aa are formed so that a junctionwith the source electrode layer 76 is to be an ohmic junction, as sameas the p-type semiconductor layers 80.

Next, a trench MOSFET according to yet another embodiment of the presentinvention is described with reference to FIG. 11. FIG. 11 is a sectionalview schematically showing a structure of the trench MOSFET according toyet another embodiment of the present invention. In FIG. 11, the samereference numerals and symbols already shown are used to designate thesame and corresponding elements, and the detailed description thereofmay not be given except for the case when there is something to add.

In this embodiment, a point is different that the gate insulating film78 in the embodiment shown in FIG. 10 are to be gate insulating film 78a which are not integrated with an insulating film protruding outside oftrenches. The rest are basically the same.

Incidentally, the structures shown in FIG. 9 to FIG. 11, in which thelonger-than-wide p-type semiconductor layers are disposed in the n-typeepitaxial layer 73 at intervals in the horizontal direction in thedrawing are generally called as super junction structures. In thisstructure, to make it to have the highest withstand voltage, and thelowest on-resistance characteristic, total volumes of the impurities ofthe p-type and the n-type in the horizontal direction at a normal unitcell shown by a line between A and Aa in FIG. 9 are made to beapproximately equal. This corresponds that the total volumes of theimpurities of the p-type and the n-type between the line between B andBa in FIG. 10 are made to be equal, and therefore, a width of the p-typesemiconductor layer 80 a (in the horizontal direction in the drawing) ispreferable to be adjusted to be narrower than that in the unit cell at apart of the MOSFET.

The present invention is not limited to the specific embodimentsdescribed here with the illustrations, but it is to be understood thatall the changes and modifications without departing from the range ofthe following claims are to be included therein.

1. A trench MOSFET, comprising: a gate electrode having a trench gatestructure; a gate insulating film formed to surround the gate electrode;an n-type diffusion layer formed to face the gate electrode via the gateinsulating film at an upper portion of the trench; a p-type base layerformed to face the gate electrode via the gate insulating film at alower portion than the upper portion of the trench; an n-type epitaxiallayer locating to face the gate electrode via the gate insulating filmat a further lower portion than the lower portion of the trench; a metallayer formed departing from the trench in parallel with a depthdirection of the trench, penetrating the n-type diffusion layer and thep-type base layer to reach the n-type epitaxial layer; and a p-typelayer with higher impurity concentration than the p-type base layer,locating to be in contact with the p-type base layer and the metallayer.
 2. A trench MOSFET as set forth in claim 1, wherein a depth ofthe metal layer is shallower than a depth of the trench.
 3. A trenchMOSFET as set forth in claim 1, wherein a depth of the metal layer isshallower than a depth of a deepest portion of the p-type base layer. 4.A trench MOSFET as set forth in claim 1, wherein a depth of a deepestportion of the p-type base layer is deeper than depths of the trench andthe metal layer.
 5. A trench MOSFET as set forth in claim 1, whereinside surfaces of the metal layer in a depth direction is not in contactwith the n-type epitaxial layer.
 6. A trench MOSFET as set forth inclaim 1, further comprising: an n-type layer with higher impurityconcentration than the n-type diffusion layer, locating between then-type diffusion layer and the metal layer.
 7. A trench MOSFET,comprising: a gate electrode having a trench (first trench) gatestructure; a gate insulating film formed to surround the gate electrode;an n-type diffusion layer formed to face the gate electrode via the gateinsulating film at an upper portion of the first trench; a p-type baselayer formed to face the gate electrode via the gate insulating film ata lower portion than the upper portion of the first trench; a secondtrench locating to face the first trench via the p-type base layer,respectively having an insulating film at a side wall surface and ap-type semiconductor layer at a bottom surface, and in which a conductorlayer is embedded; an n-type epitaxial layer locating to face the gateelectrode via the gate insulating film at a further lower portion thanthe lower portion of the first trench, and also locating at an oppositeside of the second trench from a side in which the p-type base layer islocated; and a metal layer formed to contact at an upper surface of then-type epitaxial layer at the opposite side portion of the second trenchfrom the side where the p-type base layer is located.
 8. A trench MOSFETas set forth in claim 7, wherein the conductor layer of the secondtrench is p-type polycrystalline silicon.
 9. A trench MOSFET as setforth in claim 7, wherein the conductor layer of the second trench is ametal.
 10. A trench MOSFET as set forth in claim 7, wherein theconductor layer of the second trench is in contact and conducted withthe metal layer at an upper surface thereof.
 11. A trench MOSFET as setforth in claim 7, wherein the insulating film at the side wall surfaceof the second trench is also extendedly provided at an upper surface ofthe conductor layer of the second trench to isolate between theconductor layer and the metal layer.
 12. A trench MOSFET as set forth inclaim 7, wherein the p-type semiconductor layer at the bottom surface ofthe second trench is in contact with the p-type base layer.